1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly, the present invention relates to an electrophotographic photosensitive member which has an intermediate layer interposed between an electroconductive support and a photosensitive layer.
2. Related Background Art
In a Carlson type electrophotographic photosensitive member, generally the stability of dark-portion potential and light-portion potential is of great importance in order to form images with a constant image density and without a defect in repeated charging and exposure.
To improve the stability, it is suggested to provide, between a support and a photosensitive layer, an intermediate layer which functions to improve the ability of charge injection from the support to the photosensitive layer, to improve adhesion between the support and the photosensitive layer, to improve coating characteristics of the photosensitive layer, and so on.
In recent year, a variety of "function-separation type photosensitive members" are reported in which the photosensitive layer has a lamination structure comprising a charge generation layer and a charge transport layer. In such photosensitive members, the charge generation layer is usually made in a form of an extremely thin layer, for example, in a thickness of about 0.5 .mu.m. The irregularity of the film thickness relates closely to non-uniformity of the sensitivity of the photosensitive member. Some of the major causes of the irregularity of the film thickness are a defect, a scratch, or soiling on the surface of the support. Accordingly, an intermediate layer is considered to be highly necessary.
As the layer provided between the photosensitive layer and the support, heretofore known are polyamides (JP-A-46-47344, JP-A-52-25638) (The term "JP-A" as used herein means "unexamined laid-open Japanese patent application"), polyesters (JP-A-52-20836, JP-A-54-26738), casein (JP-A-55-103556), polypeptides (JP-A-53-48523), polyvinyl alcohols (JP-A-52-100240), polyvinylpyrrolidones (JP-A-48-30936), vinyl acetate-ethylene copolymers (JP-A-48-26141), maleic anhydride ester polymers (JP-A-52-10138), polyvinylbutyrals (JP-A-57-90639, JP-A-58-106549), quaternary-ammonium-containing polymers (JP-A-51-126149, JP-A-56-60448), ethylcellulose (JP-A-55-143564), and so on.
However, in the photosensitive member which employs such a material as the intermediate layer, its potential is liable to be affected by temperature and humidity of the environment, whereby constantly stable potential characteristics and image quality could not always be attained owing to the dependency on environmental conditions.
For example, in the case where the photosensitive member is used repeatedly in an electrophotographic apparatus of positive development type at a low temperature and a low humidity, the intermediate layer comes to have a high resistance, and the light portion potential and the residual potential are made to rise and fogging occurs in the copied image because of residual electric charge remaining in the intermediate layer. On the other hand, in the case where such a photosensitive member is used in an electrophotographic printer of a reversal development type, problems arise that the image density is low and a constant density of the image cannot easily be attained.
On the contrary, at a high temperature and a high humidity, the intermediate layer comes to have an inferior barrier function owing to decrease of resistance, and carrier injection from the support side and decrease of dark portion potential are caused. Consequently, in an electrophotographic apparatus of positive development type, the density of the copied image becomes lower at a higher temperature and a higher humidity, while in a printer of a reversal development type electrophotography employing such a photosensitive member, a problem arises that image is liable to have black-spot defect and fogging.
In particular, in an electrophotographic photosensitive member of a lamination type in which the photosensitive layer is formed by sequentially laminating a charge generation layer and a charge transport layer, the potential is liable to become lower owing to increase of carrier injection from the support side, and slight lowering of barrier function of the intermediate layer tends to cause fogging in printers of reversal development type, because the charge generation layer containing a charge-generating substance in a high concentration is placed in contact with the intermediate layer.
To solve such problems, intermediate layers are reported which comprise a dispersion system of an electroconductive powdery material in a relatively highly resistant binder resin, such as an electroconductive powdery material in a polyester-polyurethane (JP-A-61-163346), titanium (IV) oxide or Sn (II) oxide in an acryl-polyurethane (JP-A-62-280863), carbon black in acryl polyol isocyanate (JP-A-62-115467), etc. In such systems, the variation of characteristics caused by variation of temperature and humidity can be decreased. However, in the system, a resin portion of high resistance and a powder portion of high conductivity are mingled, which raises problems, in reversal development type printers, that the charge injection from the support side to the photosensitive layer is liable to become non-uniform, and the potential tends to fall in a minute portion, and thereby black-spot defects in an image are caused.
Further, in repetitive use of the photosensitive member employing such an intermediate layer, at a higher processing speed of the electrophotographic process, electric charge accumulates disadvantageously at the resin portion of high resistance. Then, there is the problem that the residual potential is raised.